In the past, ZnCdS:Ag,Cl phosphor and the like have been mainly used as red phosphors, for the reason of being chemically stable or the like. However, the use of Cd has been restricted due to environmental problems, and the like, and new red phosphors that do not contain Cd have been developed.
For example, Patent Document 1 and Patent Document 2 disclose red phosphors containing calcium sulfide as a host material, Eu as a luminescent center (activator), and Mn, Li, Cl, Ce, Gd or the like as a sensitizer (co-activator).
Furthermore, Patent Document 3 discloses, as a red phosphor with which high color purity as well as satisfactory luminance and efficiency can be obtained even under low speed electronic excitation, a red phosphor represented by any one of general formulas: (Ca,Sr)S:Eu,A,F . . . (1), (Ca,Sr)S:Eu,Rb,F . . . (2), and (Ca,Sr)S:Eu,A,Rb,F . . . (3) (provided that A in the above formulas (1) to (3) contains at least one selected from Al, Ga and In at a content of 0.01 mol % to 5 mol %, and Rb at a content of 0.01 mol % to 2 mol %).
Patent Document 4 discloses a red phosphor represented by general formula: (Ca1-xSrx)S:Eu,In (provided that x in the formula represents 0 or 1), which contains In at a content of 0.05 mol % to 4.0 mol % relative to the total number of moles of Ca and Sr.
Patent Document 5 discloses an orange-colored phosphor that is excited by light in the region from near-ultraviolet to visible light, which has the same monoclinic crystalline structure as that of Eu2SiS4, and is characterized by being represented by general formula: (CaBa)1-xEuxSiS4 in which the Eu concentration is designated as x.
Patent Document 6 discloses, as a method for improving moisture resistance of a red phosphor based on alkaline earth metal sulfides such as strontium sulfide, barium sulfide and calcium sulfide, a method of dispersing red light emitting phosphor particles in an anhydrous polar solvent such as an alcohol, which contains a reactive fluoride at a low concentration, and thereby providing particles with a transparent fluoride coating.